Possible fuel indicator problem

Hello,

Just a week ago I have bought mk1 SLX Octavia 2.0i (AQY engine) and apparently I have a problem with fuel indicator on my dashboard, or maybe not.

On first day, I had exactly 1/2 of petrol in my tank (according to my fuel indicator) and had driven for about 80 km of city driving and the fuel indicator was pointing less than 1/4 fuel left, which was very disappointing for me because it indicates fuel consumption over 15 l/100 km. During this short period of driving (couple of days) I noticed that the needle skips a bit (couple of times) and reallocate its position (for a few mm).

I have refilled my thank to 3/4 of fuel indicator and start testing fuel consumption again and it was pretty much the same situation.

Yesterday, I was installing aftermarket HID on my car and accidentally loosen the + battery terminal (reset all the tacho data in my car) and now a fuel indicator indicates almost the very same level of fuel like it was before (3/4). Please note that I have no errors on my dashboard and that I have driven my car for 60 km from last refill for testing (before I started with HID installation).

This car has 167 k mileage and it’s in good condition, OEM oxygen sensor was replaced three days ago, air filter and everything else (timing belts, oil, etc), except NGK spark plugs. The car was not driven much for past three years (only during one month in summer) and it was laying in garage all the time and I now that it needs a good run to start breathing again, but I’m not sure whom to trust..a common sense or the fuel indicator

Down below you can find exhaust emission (they are not 100% correct according to Auto Data).

First picture is at idle speed and second is at 2500 rpm, like it suppose to be during emission test.

I’m not feeling comfortable not knowing how much fuel do I have in my tank.

Any suggestion pls?

Thx in advance..